1. Field of the Invention
The invention relates to internal combustion engines and, more particularly, relates to a system and method for controlling operation of an engine equipped with an exhaust gas recirculation (EGR) system.
2. Discussion of the Related Art
Countries worldwide are implementing ever-stricter emission(s) standards for diesel and other internal combustion engines. Past and some current standards for oxides of nitrogen (NOx), hydrocarbon (HC), and particulate emissions have been met through various improvements to engine design, advancements in fuel injection equipment and controls, etc. However, many of these techniques are incapable of meeting stricter emission standards that are being implemented or will soon be implemented by the United States and many other countries. Exhaust gas recirculation (EGR) is therefore becoming an increasingly important weapon in the war against emissions.
EGR systems have been used for decades to reduce NOx emissions and, as now developed, have been successfully applied to modem gasoline engines to meet past and current emission regulations. Because of the tightening NOx standards for compression ignition (diesel) engines, EGR systems are currently being investigated for application to diesel engine emission systems for reduction of NOx. However, application of EGR systems to diesel and other lean burn engines presents several distinct challenges. For instance, the direct recirculation of hot exhaust gases to the air intake system of a diesel engine increases air intake manifold temperature, increasing hydrocarbon emissions and particulate levels due to deterioration in the combustion process. In addition, soot and other particulates in the EGR system accumulate in the aftercooler and other components of the engine's intake and exhaust systems, decreasing the effectiveness of those components and shortening their useful lives. Moreover, unlike in a throttled otto cycle engine, an unthrottled diesel engine often experiences an insufficient differential pressure across the EGR line to generate an EGR flow sufficient to obtain an optimal EGR mass fraction in the air/EGR mixture inducted into the engine.
Some of the problems associated with attempting to reduce emissions in a diesel engine through EGR, and proposed solutions to them, are discussed, e.g., in U.S. Pat. No. 5,440,880 to Ceynow, U.S. Pat. No. 5,806,308 to Khair, and U.S. Pat. No. 6,301,887 to Gorel. For instance, the Gorel patent discloses a so-called low pressure EGR system for a turbocharged diesel engine. The Goret EGR system includes an exhaust particulate filter that is located downstream of and in fluid communication with the outlet of the turbocharger turbine for removing particulate matter from the exhaust gases. It also includes a low-pressure EGR line that extends from an inlet located within the main exhaust particulate filter to an outlet located upstream of the turbocharger compressor and downstream of the engine's air filter. An EGR valve, an EGR cooler, and an EGR return are located in series within the EGR line. In addition, an EGR pick-up unit is located at the inlet of the EGR line within the main particulate filter. It has an internal particulate filter to remove particulates from the EGR stream.
Solutions proposed by the Gorel patent and others solve some of the problems discussed above to the extent that it is now possible to implement a practical EGR system in a diesel engine on either an original equipment manufacturer (OEM) or an aftermarket basis. However, the controls of prior EGR equipped engines do not take full advantage of EGR when attempting to reduce emissions or otherwise optimize combustion control.
For instance, an increasingly popular technique for reducing emissions is to optimize engine operation based on excess air or “lambda.” Lambda is usually defined as the ratio of total air available for combustion during a particular combustion cycle to that required for stoichiometric combustion, i.e., that required to burn all of the fuel during that cycle. If lambda drops below a minimum threshold, the reduced oxygen level in the combustion chamber increases NOx and other emissions to unacceptable levels. On the other hand, if lambda rises above a maximum acceptable threshold, misfire can occur, resulting in excessive, unwanted emissions and sharply decreased thermal efficiency. Optimum lambda varies with speed, load, and other factors. Characteristics that are controlled to optimize lambda include fuel supply quantity, charge pressure or manifold absolute pressure (MAP), and air charge temperature (ACT).
Of course, oxygen is the only reactive constituent of air. The remaining constituents, principally nitrogen, are largely inert. Lambda based controls assume that the oxygen concentration in the combustion mixture is equal to the oxygen concentration in the ambient atmosphere, i.e., 21% on a mole fraction basis, and then base their calculations on that assumption. This assumption is incorrect in EGR equipped engines. The recirculated exhaust gases contain little or no oxygen and, when mixed with ambient air, produce an intake mixture that has substantially less oxygen on a mole fraction basis than ambient air. Lambda based controls therefore overestimate the reactability of the combustion mixture, leading to inaccurate calculations and resultant inferior controls. Other standard combustion control strategies similarly fail to adequately take the oxygen concentration reducing effects of EGR into account.
EGR is also relatively heavily laden with water vapor, which is a major combustion product. The mixing of EGR with intake air therefore introduces substantial quantities of water vapor into the resultant intake mixture. This water vapor introduction has two effects, one potentially beneficial and one potentially harmful, neither of which has been adequately addressed by the prior art.
First, the inventors have discovered that the water vapor in the intake mixture can have the same effect as water injection, which is widely-used in diesel engines to reduce the flame temperature in the combustion chamber for NOx reduction purposes. No known system takes this effect into account when adjusting engine operating characteristics such as ignition timing and lambda. Nor does any known system actively control engine operation to obtain a specific desired water vapor concentration dependent parameter in the intake mixture.
Second, under some engine operating conditions, the water vapor may condense after it is mixed with ambient air. This condensation can lead to accelerated corrosion of downstream components of the air intake system. Some systems attempt to prevent condensation by removing at least some water vapor from the exhaust stream in an aftercooler located upstream of the air/EGR mixing device. However, as should be apparent from the preceding paragraph, the removal of more water vapor than is required to avoid condensation results in reduced NOx reduction effectiveness due to the unnecessarily low moisture concentration of the intake mixture.
In light of the foregoing, it should be apparent that the need has arisen to optimize the combustion control of an internal combustion engine based on the actual oxygen concentration in the intake gas stream or a parameter indicative of the oxygen concentration.
As also should be apparent from the foregoing, the need has additionally arisen to take advantage of the water content in an EGR stream to reduce NOx emissions, preferably while still preventing condensation in the engine's air intake system.